1. Field of the Invention
The present invention relates to a light emitting diode with surface emission whose structure is adapted for supressing the radiation re-emitted by the substrate.
A light emitting diode in accordance with the invention comprises a re-emitting layer made from a material close to that of the emitting layer, the re-emitting layer absorbing the energy directed towards the substrate so as to re-emit it at a wave length close to that of the emitting layer.
2. Description of the Prior Art
A light emitting diode is a component formed essentially by an active layer and at least one confinement layer; the whole is supported by a substrate and may further comprise other layers for smoothing, making contact . . . etc, which will not be described in detail in the rest of this text, for they do not come into the study of the invention. A light emitting diode uses the spontaneous emission from the radiative recombination of electron-hole pairs created by the injection of carriers into a forward biased junction. The spontaneous radiation is isotropic.
Two cases occur.
In a light emitting diode with surface emission, whose substrate is opaque to the emitted wave length, the optical spectrum associated with the emission comprises one or two more or less distinct peaks depending on the nature of the active layer. One of the peaks comes from the substrate which absorbs the energy emitted by the active layer and re-emits it at a wave length different from that of the active layer.
If, on the other hand, the substrate is transparent to the emitted wave length, it does not re-emit since it does not absorb, but about a half of the emitted light is lost: that which exits through the rear face of the diode, i.e. through the substrate.
Because of the substantially lambertian distribution of the emitted beam, the efficiency of a light emitting diode is rather mediocre, especially when coupled with an optical fiber. In order to increase the efficiency of a light emitting diode and so as to obtain a purer spectrum, the invention provides for the addition to the structure of the light emitting diode of a layer, placed in the rear path of the emitted light, of a nature very close to that of the active layer, but with forbidden band energy less than those of the confinement layers and of the active layer: this additional layer absorbs the radiation emitted at a wave length .lambda. by the active layer and re-emits it by photoluminescence at a wave length very close to .lambda.+.DELTA..lambda.. Thus, the optical power emitted is increased by the photoluminescent emission, and the optical spectral line is slightly increased, but the parasite emission peak by the substrate is considerably reduced, even cancelled out in some cases.